1. Field of the Invention
This invention relates to musical tone synthesis and in particular is concerned with an improvement for producing several tone variations from stored musical waveforms.
The most obvious method to imitate an acoustic musical instrument is to record the sound and to replay the recording in response to an actuated keyswitch in an array of keyswitches. An advantage to a musical tone generation system using a stored replica of a musical waveform is the ability to closely approximate the tone of an orchestral type acoustical musical instrument. One of the primary drawbacks in the implementation of this type of electronic tone generation lies in the very large numbers of data points that must be stored in a memory. For a true imitation a waveshape must be stored for each different sound and for each of the 61 keyboard switches that span the standard range of an electronic organ keyboard. Some measure of economy in the waveshape memory size requirement has been made by using a single recording for several contiguous musical notes. This economy is based upon the tacit assumption that the waveshape for the imitated acoustic musical instrument does not change markedly between several contiguous successive notes.
Electronic musical tone generators that operate by playing back recorded musical waveshapes stored in a binary digital data format have been given the generic name of PCM (Pulse Code Modulation). The name "sampled waveforms" has also been applied to the same generic systems. A musical instrument of the PCM generic type is described in U.S. Pat. No. 4,383,462 entitled "Electronic Musical Instrument." In the system described in the patent, the complete waveshape of a musical tone is stored for the attack and decay portions of the musical tone. A second memory is used to store the remainder of the tone which comprises the release phase of the musical tone. The sustain phase of the musical tone is obtained by using a third memory which stores only points for a single period of a waveshape. After the end of the decay phase, the data stored in the third memory is read out repetitively and the output data is multiplied by an envelope function generator to create the amplitude variation for the sustain and release portions of the generated musical tone.
Because of the large amount of memory required for a stored waveform PCM musical tone generation system, it is desirable to employ techniques that can generate a variety of tones from the original set of stored waveforms corresponding to the waveform of a particular selected acoustic musical instrument.
It is an object of the present invention to generate an ensemble-like tonal effect with an economical system logic.
It is a further object of the present invention to vary the phase shift of a secondary waveshape in a fashion which is adaptive to the temporal variations in the fundamental frequency of the stored musical waveform.